Chronic periodontitis is an inflammatory disease of the supporting tissues of the teeth leading to resorption of alveolar bone and eventual tooth loss. The disease is a major public health problem in all societies and is estimated to affect up to 15% of the adult population with severe forms affecting 5-6%.
The development and progression of chronic periodontitis has been associated with specific Gram-negative bacteria in subgingival plaque. The presence of Porphyromonas gingivalis in subgingival plaque has been strongly associated with disease.
The persistence of P. gingivalis in subgingival plaque from periodontitis patients after treatment (scaling and root planing) has been reported to be significantly associated with progressive alveolar bone loss. Furthermore an increase in P. gingivalis cell numbers in subgingival plaque has been shown to correlate with disease severity as measured by attachment loss, periodontal pocket depth and bleeding on probing.
Oral infection with P. gingivalis has been shown to induce periodontal bone loss in mice, rats and non-human primates. In addition, there has been increasing linkage of periodontal disease, and of P. gingivalis infection, with cardiovascular diseases and certain cancers.
A number of virulence factors have been reported to contribute to the pathogenicity of P. gingivalis including; LPS, fimbriae, hemagglutinin, hemolysin and extracellular hydrolytic enzymes (especially the Arg-X and Lys-X specific proteinases), otherwise known as “P. gingivalis trypsin-like enzymes”.
The magnitude of the public health problem is such that there is a need for an antiserum, particularly specific antibodies that provide a strong protective response to P. gingivalis infection and means for providing same.
One problem has been that it is not clear how to obtain a strong protective response to P. gingivalis infection where there are a plethora of virulence factors to select from.
The relative immunogenicity of epitopes amongst virulence factors is not well understood, nor is the relative immunogenicity of epitopes on a given factor, particularly where it is not clear as to whether further epitopes remain to be identified.
One particular problem has been that many virulence factors are formed from multiple domains and are difficult to express so as to present a conformation approaching that found on P. gingivalis. Further, when these domains are expressed as discrete units i.e. in isolation of other virulence factor domains, they tend to fold into a conformation distinguished from that found on P. gingivalis. 
Further, of the many different options for modifying the immunogenicity of a virulence factor it is not clear which would be most likely to provide for a protective immune response.
In work leading to the present invention the inventors have identified peptides having an amino acid sequence that is the same as, or that shares homology with, an amino acid sequence that forms a region of a P. gingivalis trypsin-like enzyme, said region defining a site in said enzyme for cleavage of a peptide bond located C-terminal to Lys or Arg in a peptide containing Lys or Arg, and incorporated such a peptide into a chimeric or fusion protein which, when used as a vaccine, provides better protection against periodontal tissue destruction than purified proteinase-adhesin complex formed from native P. gingivalis trypsin-like enzyme or killed whole cells.